Multiple threshold voltages (VT) are needed for microprocessors. The VT of long-channel field effect transistors (FETs) tracks flat-band voltage VFB. For a metal-oxide semiconductor (MOS) capacitor, VFB=φM−χ, where φM is metal work function, and χ is electron affinity.
Multiple VT can be achieved using multiple gate metals (multiple φM). However, this approach is complicated because finding multiple gate metals that satisfy the VT requirement is not easy.
Therefore, improved techniques for achieving multiple VT FETs would be desirable.